Does your mouthwash actually kill the bacteria living in your mouth? Alpha streptococcus is one of the most common bacteria found there. Different mouthwash brands claim to fight germs, but their ingredients vary widely. You swab your mouth to isolate alpha streptococcus on a blood agar plate. Then you grow the bacteria on Mueller Hinton plates and apply measured doses of different mouthwashes. After overnight incubation, you measure the death zone (the clear ring where bacteria could not survive) around each mouthwash sample. The larger the death zone, the more effective that mouthwash is at killing alpha streptococcus.